In acoustic well logging, it is customary to measure the compressional wave velocity of earth formations surrounding boreholes. A conventional compressional wave velocity logging system includes a cylindrical logging sonde suitable to be suspended downhole in the borehole fluid, a source connected to the sonde for generating compressional waves in the borehole fluid, and one or more detectors connected to the sonde and spaced apart from the compressional wave source for detecting compressional waves in the borehole fluid. A compressional wave in the borehole fluid generated by the source is refracted into the earth formation surrounding the borehole. It propagates through a portion of the formation and is refracted back into the borehole fluid at a point adjacent to the detector and is then detected by the detector. The ratio of the distance between the source and detector to the time between generation and detection of the compressional wave yields the compressional wave velocity of the formation. The distance between source and detector is usually fixed and known so that measurement of the time between compressional wave generation and detection is sufficient to determine the compressional wave velocity of the formation. For better accuracy, such distance is usually much greater than the dimensions of the source or detector. Information important for production of oil and gas from subterranean earth formations may be derived from the compressional wave velocities of such formations.
When a compressional wave generated by a compressional wave source in the borehole fluid reaches the borehole wall, it produces a refracted compressional wave in the surrounding earth formation as described above. In addition, it also produces a refracted shear wave in the surrounding earth formation and guided waves which travel in the borehole fluid and the part of the formation adjacent to the borehole. Part of such shear wave is refracted back into the borehole fluid in the form of a compressional wave and reaches the detector in the logging sonde. The guided waves are also detected by such detector. Any wave that is one of the three types of waves detected by the detector may be called an arrival: the compressional waves in the borehole fluid caused by refraction of compressional waves in the formation the compressional wave arrivals, those caused by refraction of shear waves in the formation the shear wave arrivals, and those caused by guided waves the guided wave arrivals. Thus, the signal detected by the detector is a composite signal which includes the compressional wave arrival, the shear wave arrival and the guided wave arrivals. In earth formations compressional waves travel faster than shear waves and shear waves in the formation usually travel faster than the guided waves. Therefore, in the composite signal detected by the detector, the compressional wave arrival is the first arrival, the shear wave arrival the second arrival, and the guided wave arrivals the last arrivals. In measuring the compressional wave velocity of the formation, the time interval between generation of compressional waves and detection of the first arrival detected by the detector gives the approximate travel time of the refracted compressional wave in the formation. Hence the later shear wave and guided wave arrivals do not affect measurement of compressional wave velocity of the formation.
In addition to traveling over a vertical distance in the formation approximately equal to the distance between the source and detector, the compressional wave also travels over short distances in the fluid. The extra time required to travel such short distances introduces errors in the velocity log. To reduce such errors, conventional logging devices employ at least two detectors spaced vertically apart along the borehole from each other. The time interval between detection by the two detectors is measured instead of the time interval between transmission and detection. The ratio between the distance between the two detectors and such time interval yields the compressional wave velocity. Since the compressional wave travels over approximately equal short distances in the borehole fluid before reaching the two detectors, the time interval between detection by the two detectors is a more accurate measure of the actual travel time in the formation. Therefore, using two detectors and measuring the time between detection by the two detectors yield a more accurate compressional wave velocity. Other spurious effects such as borehole-size changes and sonde tilt may be reduced by conventional devices. One such device is described in Log Interpretation, Volume 1--Principles, Schlumberger Limited, New York, N.Y. 10017, 1972 Edition, pages 37-38.
It is well known that shear wave velocity logging may also yield information important for production of oil and gas from subterranean earth formations. The ratio between the shear wave velocity and compressional wave velocity may reveal the rock lithology of the subterranean earth formations. The shear wave velocity log may also enable seismic shear wave time sections to be converted into depth sections. The shear wave log is also useful in determining other important characteristics of earth formations such as porosity, fluid saturation and the presence of fractures.
The conventional compressional wave logging source and the compressional waves it generates in the borehole fluid are symmetrical about the logging sonde axis. When such compressional waves are refracted into the surrounding earth formation, the relative amplitudes of the refracted shear and compressional waves are such that it is difficult to distinguish the later shear wave arrival from the earlier compressional wave arrival and from the reverberations in the borehole caused by refraction of the compresional wave in the formation. Therefore it is difficult to use a conventional symmetrical compressional wave source for logging shear wave velocity. Correlation techniques have been employed to extract the shear wave arrival from the full acoustic wave train recorded. Such techniques, however, usually require processing of data by using a computer so that shear wave velocities cannot be logged on line. It may also be difficult to extract the shear wave arrival if it is close in time to the compressional wave arrival.
Asymmetric compressional wave sources have been developed for logging shear wave velocity. Using such sources, the amplitude of the shear wave arrival may be significantly higher than that of the compressional wave arrival. By adjusting the triggering level of the detecting and recording systems to discriminate against the compressional wave arrival, the shear wave arrival is detected as the first arrival. It may thus be possible to determine the travel time of shear waves in the formation and therefore the shear wave velocity. Asymmetric sources are disclosed by Angona et al, European Patent Application No. 31989, White, U.S. Pat. No. 3,593,255, and Kitsunezaki, U.S. Pat. No. 4,207,961.
Angona et al disclose a bender-type source which comprises two circular piezoelectric plates bonded together and attached to a logging sonde by their perimeters. When voltage is applied across the two piezoelectric plates, the center portion of the circular plates will vibrate to create a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. The two compressional waves will interfere to produce a shear wave in the earth surrounding the borehole.
It is explained in the last paragraph of page 8 of Angona et al. that the shear wave so produced has an amplitude which "is at a maximum in a plan of the operating faces of the bender-type transducer and falls off as a cosine function until it reaches a minimum after displacement through an angle of 90.degree.." Such a shear wave, having amplitude which varies as a cosine function of azimuthal angle, will sometimes be referred to herein as a "dipole" shear wave. The dipole-shear wave produced by the Angona et al. device has frequencies which are inversely related to the diameter of the circular plates. Such diameter cannot exceed the diameter of the logging sonde which in turn is limited by the diameter of the borehole. Because of this limitation, the shear waves generated by the bender-type source disclosed by Angona et al will have a limited frequency range, and the source may be inefficient for generating low frequency shear waves.
White discloses an asymmetric source comprising two piezoelectric segments each in the shape of a half hollow cylinder. The two segments are assembled to form a split cylinder. The two segments have opposite polarization and electrical voltage is applied to each segment, causing one segment to expand radially and simultaneously causing the other segment to contract radially, thereby producing a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. The two compressional waves will interfere to produce a shear wave in the surrounding earth formation. The frequencies of the shear wave so generated are inversely related to the radius of the split cylinder. Since such radius cannot exceed the radius of the logging sonde which in turn is limited by the radius of the borehole, White's device is not suitable for generating low frequency signals for the purpose of shear wave logging.
In Kitsunezaki, coils mounted on a bobbin assembly are placed in the magnetic field of a permanent magnet and current is passed through the coils to drive the bobbin assembly. The movement of the bobbin assembly ejects a volume of water in one direction and simultaneously sucks in an equivalent volume of water in the opposite direction, thereby generating a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. Kitsunezaki's asymmetric source, however, cannot be driven at high frequencies or with sufficient power required for harder formations. Also it cannot operate at great depths or under great pressures.
In another type of shear wave logging source, instead of coupling the source to the borehole wall through the medium of the borehole fluid, the source is either coupled directly to the borehole wall or through mechanical means such as mounting pads. Such shear wave logging sources are disclosed in Erickson et al, U.S. Pat. No. 3,354,983 and Vogel, U.S. Pat. No. 3,949,352.